Porphyria cutanea tarda (PCT) is recognized worldwide as the most common form of human porphyria. The photocutaneous disorder is caused by porphyrin overproduction in the liver which relates to diminished activity of uroprophyrinogen decarboxulase (UROD). The metabolic lesion almost invariably requires iron overload for its clnical and biochemical expression. We propose to evaluate the key role of iron in genesis of PCT using as an animal model, the hexachlorobenzene (HBC) porphyric rat, where the disturbances in hepatic heme biosynthesis closely resemble that seen in PCT. (1) We will determine in HCB rat porphyria the activity of both UROD and the porphyrin content in liver, renal and intestinal tissues which we will relate to iron content following maneuvers which selectively load or deplete these tissues of iron. (2) The effect of iron on uroporphyrinogen (UROGEN) decarboxylation will be studied both vy an in vivo/monolayer hepatocyte culture approach, and in vitor. Porphyrins will be characterized and quantitated in liver, urine and feces, and hepatic UORD activity determined in HCB-porphyric, iron-loaded, iron-depleted and control rats. Monolayer hepatocyte culture derived from these animals will allow comparions of the de novo porphyrin and heme synthesis before and after stimulation with exogenous Delta-aminolevulinic acid. The effect of iron on UROD activity in vitro will be studied under controlled conditions with O2, reductants, microsomes, products of lipid peroxidation and antioxidants as variables. (3) Porphyrinogen oxidation during UROGEN decarboxlation in vitro and in liver cell culture (HCB-porphyric hepatocytes) will be quantitated under conditions where iorn might generate oxidants directly by acting as an electron carrier in the presence of reductants and O2 or via iron-stimulated lipid perioxidation. (4) The influence of in vivo lipid peroxidation on HCB rat porphyria under conditions of iron-loading and iron-depletion will be studied. (5) We will measure the effect of iron on heme turnover in normal and HCB-porphyric rat hepatocytes in monolayer culture, and relate de nove heme syntehsis to porphyrin production. Our intent is to determine whehter any increase in hepatocyte porphyrin content is consistent with or disporportionate to substrate loading of the UROD-deficient liver cells.